The invention is generally related to the field of MOSFET transistors and more specifically to forming a MOSFET structure using a disposable gate process.
As CMOS technology continues to scale further into the sub-micron region, it becomes increasingly difficult to keep sufficiently low gate sheet resistance, low junction capacitance, and low junction depth of source/drain extensions under the gate. As transistors are scaled into the deep sub-micron region, the polysilicon gate linewidths become narrower and narrower. This increases the gate sheet resistance. Achieving low gate sheet resistance becomes difficult even when silicided polysilicon is used. The source/drain junction regions and source/drain extensions must also become shallower to avoid undesired short-channel effects and roll-off of the threshold voltage at short channel lengths. However, in the deep sub-micron region, it is difficult to achieve shallower doping profiles by conventional means such as ion implantation. Therefore, there is a need for a CMOS transistor structure that can be scaled further into the sub-micron region while maintaining sufficiently low gate sheet resistance, small junction depth, and low junction capacitance.
One of the most challenging issues facing the fabrication of sub-micron transistors is forming shallow source/drain extensions under the sidewall spacer. This problem is especially severe for the replacement gate transistor design if inner sidewalls spacers are added to decrease the linewidth of the replacement gate which is formed in the slot in an insulator left by removal of the disposable gate. There are a number of prior art inventions in which a gate is formed in a slot in an insulator by a means other than the removal of the disposable gate. A common problem with these inventions is how to place a shallow drain extender underneath sidewall spacers deposited in the slot. In one prior art example, after nitride spacers are formed on the inner walls of the slot, thermal oxide is grown in the space between the sidewalls. Next, the nitride spacers are removed and the shallow source/drain junction extenders are implanted into the spaces where the nitride spacers have been removed with the thermal oxide in the center blocking these implants. Next, the nitride spacers are reformed, the center thermal oxide removed, and replaced with a thin gate oxide. Finally, the slot is filled with the gate material. This is a very complicated process which is not very manufacturable.
The instant invention is a method of fabricating a transistor using selective n+/p+ epitaxial silicon before gate patterning to construct shallow drain extenders for transistors using the Replacement Gate Design. An embodiment of the method of fabricating a transistor comprises the steps: providing a silicon substrate with an upper surface of a first conductivity type; forming a silicon epitaxial layer of a second conductivity type on said upper surface of said silicon substrate surface; forming a disposable gate over a first defined region of said silicon epitaxial layer; forming a plurality of outer sidewall structures adjacent to said disposable gate; forming a plurality of source/drain regions adjacent to said outer sidewall structures; forming an insulator layer over said source/drain regions; removing said disposable gate without substantially removing any other exposed material; forming a plurality of inner sidewall structures adjacent to said outer sidewall structures; removing said silicon epitaxial layer that lies between said inner sidewall structures thereby exposing a portion of said silicon substrate; forming a gate dielectric over said exposed portion of said silicon substrate; and forming a gate electrode over said gate dielectric. The technical advantage of the instant invention will be readily apparent to one skilled in the art from the following FIGUREs, description, and claims.